FERRUARY 14, 1907 | 
NATURE 
349 
ITALIAN VOLCANIC ROCKS. 
LTHOUGH the Tertiary and Recent volcanic tract 
along the western side of the Apennines is classic 
ground to the geologist no less than to the historian, we 
still possess only meagre information concerning the many 
remarkable, and often unique, rock-types for which these 
Italian volcanoes have long been famous. A comprehensive 
and connected study of a large part of the assemblage by 
a well-qualified authority is therefore peculiarly welcome. 
Dr. Washington has devoted much attention to the subject, 
both before and since the publication, ten years ago, of 
his *‘ Italian Petrological Sketches.’’ 
““Comagmatic region’’ is synonymous with “ petro- 
graphical province,’’ and the author’s reasons do not con- 
vince us of the necessity of abandoning a now familiar 
term. The Roman region is defined as extending from 
Lake Bolsena to the Phlegreean Fields; and probably few 
petrologists will dissent from the proposition that the 
community of characters among the volcanic rocks of this 
region points to a real genetic relationship of the several 
Sketch Map of Italian Comagmatic Regions. 
oz Roman V =Vulsinian District Ve=Vesbian Volcano 5 = Monte Amiata 
WZYZ, wesion. Ci=Ciminian District P wanKields 6 =Tolfa 
Tuscan S =Sabatinian District I 7 =Cerveteri 
ontecatini B =Berican Hills 
Sampiglia E =Euganean Hills 
fassa Marittima Vu—Monte Vulture 
Region. L =Latian District I 
Venetian H=Hernican District 2 
Region. A =Auruncan District 3 
A ath 
Region. 
Ca=CampanianDistrict 4 =Roccastrada Et =Etna 
magmas. The author separates, though somewhat doubt- 
fully, the smaller “ Tuscan region,’’ lying farther to the 
north and west, which we hope will be the subject of a 
future memoir. It can scarcely be denied, however, that 
a certain community of characters unites all the Italian 
volcanic districts on this side of the Apennines (with Monte 
Vulture in the mountain-belt itself), the resemblance being 
emphasised by contrast with the rocks of the Euganean 
Hills on the opposite side of the main orographic line. 
The body of the memoir before us consists of two parts. 
» The first is purely descriptive, the several rock-types being 
treated in order, succinctly but thoroughly. The special 
features of this part are the quantitative element constantly 
introduced into the mineralogical descriptions, and the 
addition of a large number of new and carefully-made 
chemical analyses of the lavas. The peculiarity which has 
made the region famous in petrography is the abundance 
1 “The Roman Comagmatic Region.” By Henry S. Washington. 
Pp. vi-+-199- (Washington: Carnegie Institution, 1900.) 
NO. 1946, VOL. 75] 
and variety of leucite-bearing rocks. The non-leucitic types 
are for the most part of trachytic affinities, though with a 
proportion of soda-lime-felspar which caused the author 
(in his former papers) to distinguish them under the names 
vulsinite and ciminite. 
The second part of the memoir, discussing the mutual 
relations of the associated rocks, is headed *‘ Petrology ”’ 
(the first part being ‘‘ Petrography ’’). It would seem more 
convenient to use the name petrology for the whole science 
of rocks, including the descriptive branch (petrography) 
and the rational. The author gives an interesting dis- 
cussion of the facts which he has brought together, and 
touches on the genetic problems which underlie those facts. 
In particular, he attempts a calculation of the average 
composition of the magmas for the several districts and 
for the whole region. In the central part of the region 
all the lavas carry leucite, basic leucite-tephrites and 
leucitites being largely predominant; while at the two 
extremities of the region the trachytic types are in greater 
force. No definite order of succession in time can be 
made out. 
While taking care to make his work intelligible to the 
ordinary petrologist, Dr. Washington employs throughout 
the methods and terminology of the Quantitative Classifi- 
cation, of which he is joint author. The memoir thus 
written does, as he claims, serve to make that system 
clearer by showing it in actual operation, and this is an 
incidental gain; but, although it is here seen at its best, 
as applied to a cognate collection of types, most of which 
possess strongly marked characteristics, we do not find our 
fundamental objections to the new classification weakened 
by a closer acquaintance with it. If a rigidly quantitative, 
and therefore artificial, classification be desirable, which 
we do not concede, it might be sought in the actual mineral 
composition of the rock (here estimated in most cases) 
rather than in the imaginary composition which is called 
the “‘norm.’’ In reading the descriptions and discussions, 
it needs no very perverse fancy to construe many sentences 
as censuring Nature for departing from the ‘‘ norm,”’ or 
commending her for approximately conforming to it; and 
this air of artificiality must somewhat discount the useful- 
ness of what is undoubtedly a very valuable monograph. 
A. H. 
INVERSION TEMPERATURES FOR AIR AND 
NITROGEN. 
HE Bulletin of the Cracow Academy of Sciences for 
December, 1906, contains a preliminary note, by Prof. 
K. Olszewski, on the determination of the temperature of 
inversion of the Joule-Kelvin effect for air and for nitrogen 
when subjected to different pressures. The apparatus used 
was similar in principle to that adopted in 1901 in deter- 
mining the inversion temperature for hydrogen, but details 
had to be modified owing to the necessity of working at 
much higher temperatures. The table which follows shows 
the inversion temperature of the gas when allowed to 
expand from the initial pressure p (expressed in kilograms 
per square centimetre) to the pressure of the atmosphere. 
Above the temperature t; a thermo-element showed a heat- 
ing effect on expansion, whilst below this temperature a 
cooling effect was observed. 
Air Nitrogen 
p ti | p tj 
160 +259 159 +243 
100 249 | 126 238 
90 244 102 | 233 
80 240 go | 228 
70 235 80 223 
60 226 | 68 217 
49 198 55 | 205 
20 | 124 30 | 163 
It is seen that the inversion temperature is a continuous 
function of the pressure, confirming the recent theoretical 
views of Witkowski and Porter. The value of the in- 
